Generally, base material, for example of high-speed steel or cemented carbide, having a hard film such as of titanium nitride or titanium aluminum nitride formed on the surface thereof by physical vapor deposition method (hereinafter, referred to as PVD method), chemical vapor deposition method (hereinafter, referred to as CVD method), or the like have been used as cutting tools and sliding parts, for which superior wear resistance and sliding property are demanded.
In particular, for application as a cutting tool, the hard film thereon should have high wear resistance and heat resistance (oxidation resistance at high temperature), and thus, titanium aluminum nitride (TiAlN), which is superior in both of these properties, has been used widely as a coating material for applications such as cemented carbide tools, the edge of which becomes heated to high temperature during cutting. The reason for such superior properties of TiAlN is that the TiAlN film is improved in heat resistance by the action of aluminum contained in the film and exhibits high wear and heat resistance consistently up to a high temperature of approximately 800° C. Various TiAlN compounds different in the composition ratio of Ti to Al have been used as the TiAlN's, but most of them have a composition at a Ti:Al atomic ratio in the range of 50:50 to 25:75, in which the TiAlN is superior in both properties.
Meanwhile, the edge, for example, of a cutting tool becomes heated occasionally to a temperature of 1,000° C. or higher during cutting. Under such circumstance, because the TiAlN film alone is not satisfactory as it is for ensuring sufficient heat resistance, an alumina layer is formed additionally on the TiAlN film for ensuring heat resistance, as disclosed, for example, in U.S. Pat. No. 5,879,823.
Alumina has various crystal structures depending on temperature, but any of the crystal structures except α-crystal structure is in the thermally metastable state. However, as in the case of a cutting tool, when the edge temperature during cutting fluctuates in a wide range from room temperature to 1,000° C. or higher, the crystal structure of alumina changes, causing problems such as cracking and delamination of film. However, alumina in the α-crystal structure once formed by CVD at a higher substrate temperature of 1,000° C. or more retains its thermally stable structure, independently of the temperature thereafter. Thus, coating of an alumina film in the α-crystal structure is considered as an effective means of adding heat resistance to cutting tools and others.
However as described above, it is necessary to heat the substrate to 1,000° C. or higher to form alumina in the α-crystal structure, which restricts the kinds of base materials applicable. It is because the base material may soften and lose its favorable qualities as a substrate for wear-resistant products depending on its kind, when exposed to a high temperature of 1,000° C. or higher. In addition, even high heat-resistant base materials, such as cemented carbides, also cause problems such as deformation, when exposed to such a high temperature. Practical temperature range for use of the hard films, such as TiAlN film, formed on a base material as an wear-resisting film is generally approximately 800° C. at the highest, and the film may decompose and lead to deterioration in wear resistance when exposed to a high temperature of 1,000° C. or higher
To overcome such problems, U.S. Pat. No. 5,310,607 reported that a composite (Al,Cr)2O3 crystal having a hardness at the same level as that of the alumina described above was obtained in a low-temperature range of 500° C. or lower. However, if the work material is a material containing iron as the principal component, Cr existing on the surface of the composite crystal film often reacts chemically with iron in the work material during cutting, causing enhanced consumption of the film and reduction of the lifetime.
Alternatively, O. Zywitzki, G.Hoetzsch, et al. reported in “Surf Coat. Technol.” (86-87, 1996, pp. 640-647) that it was possible to form an aluminum oxide film in the α-crystal structure at 750° C. by reactive sputtering by using a pulsed power supply at a high output (of 11 to 17 kW). However, it is inevitable to expand the size of the pulsed power supply for obtaining aluminum oxide in the α-crystal structure by this method.
As a method for overcoming such problems, Japanese Unexamined Patent Publication No. 2002-53946 discloses a method of using an oxide film in corundum structure (α-crystal structure) having a lattice parameter of 4.779 Å or more and 5.000 Å or less and a film thickness of at least 0.005 μm as an underlayer and forming an alumina film in the α-crystal structure on the underlayer. It was indicated there that the component for the oxide film is preferably Cr2O3, (Fe,Cr)2O3 or (Al,Cr)2O3; when the component for the oxide film is (Fe,Cr)2O3, it is more preferably (Fex,Cr(1-x))2O3 (wherein, 0≦x≦0.54); and when the component for the oxide film is (Al,Cr)2O3, it is more preferably (Aly,Cr(1-y))2O3 (wherein, 0≦y≦0.90).
The Japanese Unexamined Patent Publication No. 2002-53946 above also disclosed that it was effective to form a film of a composite nitride of Al and one or more elements selected from the group consisting of Ti, Cr, and V as a hard film and additionally as an intermediate layer an oxide film having the corundum structure (α-crystal structure) by oxidizing a film of (Alz,Cr(1-z))N (wherein, 0≦z≦0.90) and then form alumina in the α-crystal structure on the oxide film.
As described above, it is possible to form an alumina film almost only in the α-crystal structure according to the method described in the Japanese Unexamined Patent Publication No. 2002-53946, but the inventors have found that the method often causes growth to coarse crystal grains in the alumina film, from the following experiments:
According to the method described in Japanese Unexamined Patent Publication No. 2002-53946, a CrN film was formed on cemented carbide, the surface thereof was oxidized, and an alumina film was formed on the oxidized CrN film surface, and the alumina film obtained was observed.
More specifically, a sample 2 in which a CrN film was formed on a cemented carbide base material by ion plating method (AIP method) was prepared, and the sample was connected to the planetary revolving jig 4 and heated to 750° C. with the heaters 5 after the chamber 1 was evacuated to almost vacuum state in the device shown in FIG. 1.
When the sample 2 is heated to a predetermined temperature, an oxygen gas was introduced into the chamber 1 at a flow rate of 300 sccm to a pressure of approximately 0.75 Pa, and the sample 2 was oxidized while heated for 20 minutes.
Then, an alumina film was formed on the undercoat film after oxidation. The alumina film was formed by the reactive sputtering method, i.e., by heating the base material to a temperature similar to that in the oxidation step (750° C.) in an argon and oxygen environment while applying a pulsed DC power of about 2.5 kW respectively to the two sputtering cathodes 6 each with an aluminum target shown in FIG. 1. The protective alumina film was formed at a discharge condition in a so-called transition mode, while controlling the discharge voltage and the flow rate ratio of argon/oxygen by using plasma emission spectroscopy. A protective alumina film having a thickness of approximately 2 μm was formed in this manner.
The crystal structure of the alumina film was identified by analyzing the surface of the alumina film obtained by using a thin-film X-ray diffractometer. As a result, only diffraction peaks indicating α-alumina were observed as the diffraction peaks indicating alumina, confirming that the alumina film obtained by the method was an alumina film predominantly containing α-alumina.
Then, the alumina film was observed under a SEM (scanning electron microscope). The surface micrograph is shown in FIG. 2. FIG. 2 reveals that the alumina film obtained by the method is superior in crystallinity and has crystal grains definitely distinguishable, because of drastic crystal growth even at a film thickness of approximately 2 μm. The alumina film in such a surface state seems to have an increased surface roughness.
Then, the cross section of the alumina film was observed under a TEM (transmission electron microscope), and the film was subjected to an EDS analysis. The result is shown in FIG. 3.
FIG. 3 reveals that the film consists of three layers: from the base material, a CrN film, a chromium oxide (Cr2O3) layer having a thickness of 30 to 40 nm obtained by oxidation of the CrN film surface, and an alumina film. The results by electron diffraction pattern confirmed that the alumina and chromium oxides are both in the corundum structure.
The micrograph of FIG. 3 shows that the alumina crystal grains constituting the alumina film are grown larger as they become closer to the film surface. Increase in the surface roughness of alumina film due to growth to coarse crystal grains may cause problems, depending on applications. For example, when the film is applied to a cutting tool, the work material may be more likely adhered to the alumina film surface.
The present invention was completed under the circumstances above, and an object thereof is to provide a protective alumina film containing fine crystal grains of alumina in the α-crystal structure, and a method of producing the protective alumina film.